The Government wants to drop the baby bonus from $5000 per child to $5000 for a family's first child, and $3000 for subsequent children.

The change is estimated to save more than $500 million from the budget over the next four years.

While the Opposition had publicly criticised the move as bad for families, shadow treasurer Joe Hockey is said to support the saving and has reportedly vowed to fight his own party on their opposition to it.

The Opposition Leader today said the Government had indicated it would not change the baby bonus and he believed they should keep that promise.

But he would not necessarily act to restore the money to families if he won government, he said.

"If this legislation is passed by the Parliament, we give no guarantees we would be able to restore it because we are in a very dire fiscal position," he said.

Without Opposition support, the Government needs the backing of six out of seven crossbenchers in the House of Representatives to pass its changes.